1. Field of Invention
The present invention relates to a light emitting device open/short detection circuit; particularly, it relates to a light emitting device open/short detection circuit with an adjustable reference voltage to determine open/short condition.
2. Description of Related Art
FIG. 1 shows a schematic diagram of a backlight control circuit of US Publication No. 2008/0094349. Referring to FIG. 1, besides a voltage supply circuit 11, an error amplifier circuit 13, a lowest voltage selection circuit 21, and current sources CS1-CSN, the backlight control circuit 30 also includes under current detection circuits UCD1-UCDN. These under current detection circuits UCD1-UCDN detect the current conditions on the light emitting diode (LED) paths 101-10N respectively to determine whether an under current status, i.e., a “no current” or “very low current” condition, occurs. When “no current” or “very low current” condition does not occur, the voltage signals on the LED paths 101-10N pass through the UCD circuits 31-3N to the corresponding voltage comparison paths 111-11N, so that a lowest voltage selection circuit 21 receives those signals, and selects a lowest voltage among them to input to the error amplifier 13. The error amplifier circuit 13 compares the lowest voltage with a reference voltage Vref to generate a control signal 15.
When anyone or more LED paths 101-10N have no current or very low current, the UCD circuits 31-3N exclude the corresponding one or more voltage comparison paths 111-11N so that they are not valid inputs to the lowest voltage selection circuit 21, that is, the lowest voltage selection circuit 21 does not accept signals on these invalid voltage comparison paths 111-11N to avoid an error.
In the aforementioned prior art, the under current detection circuits UCD1-UCDN are generally integrated with the backlight control circuit 30 in an integrated circuit (IC) chip, and therefore the circuit can not flexibly set different criteria such as the number of the short-circuited LEDs in the LED path for the under current detection circuits UCD1-UCDN to determine a failure, and thus the circuit may be over-protected. Besides, for various applications, it is also required to detect an open-circuit condition in the LED path.
In view of the foregoing, the present invention provides a light emitting device open/short detection circuit, which may be located outside a light emitting device control circuit (which is often integrated as an IC) for flexible adjustment to optimize the detection parameters/criteria such as the number of the short-circuited light emitting devices, to broaden the applications of the light emitting device control circuit, and to simplify the circuit design of the light emitting device control circuit.